The Doppler shift of a laser has been used to measure the velocity of blood in the coronary sinus of the dog. The laser was delivered along a fine fiber-optic probe, which was also used to receive the reflected light. The Doppler shift was measured from the output of a digital spectrum analyzer. This measurement, proportional to the velocity of blood, was correlated with the blood flow measured by an electromagnetic flowmeter. Studies were performed on 112 successful maneuvers in eight consecutive anesthetized dogs. In the minimally damped electromagnetic flow traces, the correlation between coronary sinus blood flow velocity and coronary sinus blood flow was high (r greater than 0.95). This measurement will give absolute flow values only if the exact dimensions of the coronary sinus are known. The linear relationship of coronary sinus velocity and flow supports the postulate that coronary sinus dimension remains similar during normal and high flows. This technique, fiber-optic laser Doppler anemometry, is an accurate measure of relative coronary sinus blood flow. It is suitable for use in human coronary sinuses, especially to monitor the effect of interventions.